The present invention relates to an apparatus for executing surface treatment such as cleaning, modifying or the like on substrate surfaces by plasma, and a method for substrate surface treatment carried out by the substrate surface treatment apparatus.
High-packaging density has been required in the field of a mounting technique in accordance with miniaturization and multifunction of electronic devices. Consequently, connections between elements and substrates should be carried out on a remarkably fine scale, and mounting with a higher degree of reliability is being required. There currently is a method of modifying substrate surfaces by plasma, i.e., plasma treatment as one example for securing the reliability. For instance, the plasma treatment can remove an organic contaminant adhering to the substrate surface, and the bonding strength between a gold electrode and a wire in the case of wire bonding can be improved when an inorganic substance such as nickel hydroxide or the like deposited on an electrode face as a bonding face formed of copper, nickel, and gold on a printed board is removed by the sputtering action of argon plasma. Also in the case where an IC is to be bonded to a lead electrode on a polyimide film substrate via an ACF (anisotropic conductive film), the bonding strength between the polyimide film and the ACF can be improved through activation of a surface of the polyimide film substrate by irradiating oxygen plasma to the film before bonding. Moreover, the plasma treatment carried out on the substrate improves the fluidity of a sealing resin on the substrate and the adhesion between the substrate and the sealing resin.
An example of the plasma treatment method referred to above will be described below with reference to the drawing FIGS. 3-5.
FIG. 3 roughly shows the configuration of a conventional apparatus 20 for surface treatment of mounting substrates, in which a reaction chamber 1 being grounded is provided with a gas introduction port 2 and a vacuum exhaust port 3. A RF electrode 5 is arranged in the reaction chamber 1 via an insulating ring 4 to a side wall of the reaction chamber 1. The RF electrode 5 has a constitution on which a mounting substrate 6 can be placed. An opposed electrode 7 is arranged in the reaction chamber 1 so as to face the RF electrode 5 and is grounded. A RF(Radio-Frequency) is applied to the RF electrode 5 by a RF supply source 8 through a matching tuner (not shown) and a RF power supply part. O-rings (not shown) are interposed between the RF electrode 5 and the insulating ring 4 and between the insulating ring 4 and the side wall of the reaction chamber 1. For preventing the O-rings from being heated to 200° C. or higher and maintaining the reaction chamber 1 in vacuum, a cooling groove 9 where a cooling water flows is formed in the side wall of the reaction chamber 1.
The surface treatment method to mounting substrates carried out by the above-constituted surface treatment apparatus 20 will be described hereinbelow in an example in which an argon gas is used for substrates before wire bonding.
The substrate 6, before being subjected to wire bonding, is placed on the RF electrode 5. While a degree of vacuum in the reaction chamber 1 is kept to be 30 Pa with 50SCCM (standard cc/min) of the argon gas being supplied from the gas introduction port 2, a RF(Radio-Frequency) of 200 W is applied to the RF electrode 5, thereby generating plasma. Argon ions in the plasma are irradiated onto a face of the substrate 6 exposed in the plasma. The substrate 6 is formed of glass cloth epoxy resin. An electrode 10 formed on the surface of the substrate 6 is constituted of three layers of a copper layer 11 having a film thickness of 35 μm, a nickel layer 12 having a film thickness of 3 μm and a gold layer 13 having a film thickness of 0.05 μm as shown in FIG. 4. The undercoat nickel 12 is moved onto a surface of the gold 13 through a heat process or the like, whereby nickel hydroxide or the like is deposited. The nickel hydroxide is sputtered and removed by the irradiation of argon ions. The surface of the gold 13 is cleaned accordingly.
FIG. 5 is a schematic diagram of a case in which a silicon chip IC 16 is bonded via an ACF (anisotropic conductive film) 15 to a polyimide film substrate 14. As shown in FIG. 5, electrodes 18 of the IC 16 are bonded via the ACF 15 composed of a resin containing conductive particles to electrode parts 17 on the polyimide film substrate 14. A surface treatment method for the polyimide film substrate 14 having the above constitution will be described below.
The polyimide film substrate 14 is placed on the RF electrode 5. A RF(Radio-Frequency) of 200 W is applied to the RF electrode 5 while a degree of vacuum in the vacuum chamber 1 is maintained at 30 Pa with 50SCCM of an oxygen gas supplied from the gas introduction port 2. As a result, plasma is generated. Oxygen radicals or oxygen ions present in the plasma are irradiated onto a surface of the polyimide film substrate 14 exposed in the plasma. The oxygen radicals react with contamination organic substances adhering on the polyimide film substrate 14, whereby the contamination organic substances are decomposed to be sublimation compounds such as CO2 or the like and then removed. Further, functional groups such as C═O, COOH and the like are generated on the surface of the polyimide film substrate 14, activating the surface of the polyimide film substrate. The bonding strength between the polyimide film substrate 14 and the ACF 15 is improved accordingly.
In the case of polyimide film substrate 14, residual ions of chlorine or the like are left yet on the polyimide film substrate 14 when the apparatus receives the polyimide film substrate 14. The reason for this is that hydrochloric acid is used as one of components of a plating solution for forming a pattern of the electrodes 17 on the polyimide film substrate 14 by plating, and, for example, chlorine ions are left if the substrate is not fully cleaned by water after the pattern is formed. In the event that the IC 16 is connected with the use of the ACF 15 to the polyimide film substrate 14 having the residual ions, the residual ions cause corrosion and electrical failures such as ion migration, etc. As such, the plasma treatment is carried out to remove the chlorine ions.
However, if the plasma treatment is carried out on the substrate 6 before being subjected to wire bonding, not only the organic contaminant, inorganic substance, or the like, but the substrate 6 is sputtered by argon ions simultaneously. In the case of the substrate 6 formed of glass cloth epoxy resin, Br (bromine) included in the substrate 6 adheres again to the substrate after being separated from the substrate 6 by the plasma treatment. In the case of the Br adhering to the electrode 10, the trouble is that the Br adhering on the electrode 10 reacts with moisture in the air and becomes HOBr or HBr when the substrate 6 is exposed to the atmosphere, which causes corrosion of the electrode 10.
When the plasma treatment is carried out with the aim of removing residual ions adhering to the polyimide film substrate 14, since there is no means for observing whether or not the residual ions are actually removed, the plasma treatment may be executed to an excessive stage in order to perfectly remove the residual ions. Thus the trouble is that the excessive plasma treatment may also damage the polyimide film substrate 14.